The separation of various components found in liquids or gases may be effected in a multitude of processes, the techniques for effecting the separation including ultrafiltration or reverse osomosis. A particular example of the latter type of separation involves a desalination process in which water, which is rendered potable or suitable for other purposes, is obtained from sea water, contaminated water, brackish water or brine. This process is of especial value in areas of the world where the water found in the area is brackish or is saline in nature. The desalination of this water is necessary in order to provide large amounts of potable or relatively nonsalty water for industrial, agricultural or home use. The desalination of the water is effected by forcing the water through a reverse osmosis membrane whereby the purified water is passed through the membrane and recovered, while the contaminants or salts do not pass through the membrane, thus, in effect, being rejected by the membrane and recovered as the retenate.
A reverse osmosis membrane, in order to be utilized for such a purpose, must possess certain characteristics applicable to the process. For example, the membrane must have a very high salt rejection coefficient. In addition, another important characteristic and a problem which must be addressed when utilizing the membrane, is the ability of the membrane to be resistant to chlorine attack. The resistance to chlorine is vital to the continued use of a membrane inasmuch as chlorine-containing compounds such as free chlorine, chloroamines, chlorine dioxide or hypochlorites, in many instances, are introduced to saline water such as sea water prior to subjecting the water to purification by means of a desalination process. These compounds are added to act as a disinfectant or as an antibacterial agent as well as to prevent the fouling of the membrane during the process due tothe growth of microbiological organisms on the surface of the membrane.
Another important factor which is present in the use of a reverse osmosis membrane is that said membrane also possess a high flux characteristic, that is, the ability to pass a relatively large amount of water through the membrane at relatively low pressures. If a membrane possesses these desirable characteristics, it will be commercially feasible in its applicability to the desalination process.
Reverse osmosis membranes have been prepared and used from a wide variety of known polymeric materials. While many of these polymeric materials possess the ability of reducing the concentration of solute to where the salt rejection capability is in excess of 98%, some do not possess the necessary flux rate whereby the volume of water which is required to be produced by the membrane per unit of membrane surface is sufficient for the application of the technology.
As was hereinbefore set forth, many prior U.S. patents describe various membranes which are useful in desalination processes. For example, U.S. Pat. Nos. 3,567,632, 3,600,350, 3,710,945, 3,878,109, 3,904,519, 3,920,612, 3,951,815, 3,993,625, and 4,048,144 illustrate various semipermeable membranes prepared from polyamides. Likewise, U.S. Pat. Nos. 3,260,691, and 3,480,588 disclose coating compositions which are obtained from the condensation products of aromatic primary diamines and aromatic tricarboxylic compounds.
Inasmuch as the semipermeable membrane which is used for the desalination process should be relatively thin in nature in order to provide a desirable flux rate, it is necessary, in many instances, that the reverse osmosis membrane be composited or laminated on a porous backing support material. This porous support backing material should in itself possess certain characteristics which make it desirable for such a use. For example, the porous support material should possess pore sizes which are sufficiently large enough so that the water or permeate can pass through the support without affecting or lessening the flux rate of the entire composite. Conversely speaking, the pore size should not be large enough so that the thin composite semipermeable membrane will tend to fill up or enter into the pores, thus distorting the shape of the thin film membrane with the attendant possibility of rupturing the membrane, thus causing said membrane to lose its effectiveness in the reverse osmosis process.
In addition to the aforementioned U.S. patents, another U.S. Pat. No. 4,277,344, discloses an interfacial synthesized reverse osmosis membrane. This membrane is prepared from a cross-linked interfacially polymerized aromatic polyamide which has been prepared from an essentially monomeric polyacyl halide and an essentially monomeric arylene polyamine. The patent indicates that the monomeric arylene polyamine which is employed as one of the components in the interfacial polymerization reaction should be free of interfering substituents. The patent refers to these substituents as moieties which are capable of interfering with intermolecular amide-forming condensation reactions, and points out that such interference is generally steric, polar, and/or chemically reactive in nature. An example of steric interference or steric hindrance would be the location of a substituent other than hydrogen, even as small as a methyl group, on a ring position adjacent to an amine substituent on the arylene polyamine reactant. An example of polar/steric interference would be the presence of polar substituents, such as methoxy groups, on the arylene polyamine, particularly on a ring position adjacent to an amine substituent; while an example of chemical interference would be the location of an acyl-reactive substituent on the arylene polyamine or an amine-reactive substituent on the polyacyl halide. The patent further states that such chemical interfering substituents would lead to the formation of internal esters, internal amides, internal salts, or the like, or another possible consequence which would be attendant upon the performance of these moieties would be an unpredictable effect on cross-link densities. U.S. Pat. No. 4,086,215 teaches the use of asymetric free-standing polyamides which have been cross-linked with metal salts for use in permselective membrane applications. The preparation of these membranes involves three separate operations comprising: (1) the synthesis of linear soluble polyamides in solutions, (2) the preparation of asymmetric freestanding protomembranes from these polyamide solutions, and (3) the generation of active membranes by cross-linking the protomembranes with solutions of metal salts. The polyamides which are utilized in this patent consist of three components, namely, a diamine component bearing a pendant carboxylic acid or sulfonic acid group, a diamine component having a neutral aromatic or aliphatic nucleus, and a diacyl chloride or dianhydride component. The patentee states that the first component is essential to the performance of the membrane and it is this component which enables the protomembrane to be cross-linked with the metal salt and thus achieve the desired membrane performance. As will hereinafter be shown in greater detail the membrane of the present invention which is utilized to effect a desalination process and which is chlorine-resistant will comprise an ultra thin active layer consisting of only two components, namely, a diaryl methylene monomer containing at least 1 amine radical on each aryl nucleus, and an aromatic carboxylic acid chloride monomer. The amine monomers of the membrane of the present invention do not require the presence of a carboxylic acid or sulfonic acid pendant group in order to achieve the desired membrane performance, nor does the membrane system of the present invention require cross-linking with metal salts in order to achieve the desired performance.
In addition to these patents, U.S. Pat. Nos. 4,378,400 and 4,512,893 also describe the use of free-standing asymmetric polyimides for membrane application. The patents employ diaminodiphenyl methane in polyimide membranes for use in gas separations. However, these patents do not suggest the use of an aromatic tricarboxylic acid chloride reactant as one component of the desired membrane. Furthermore, polyimides and polyamides represent different chemical classes, each one having its own unique properties.
Another U.S. Pat. No. 4,529,646, discloses a process for preparing composite membranes. The process involves forming a porous polysulfone followed by quenching the polysulfone membrane in an aqueous solution of m-phenylenediamine, and thereafter reacting the m-phenylenediamine on the polysulfone membrane with either trimesoyl chloride or cyclohexane 1,3,5-tricarbonyl chloride in a water immiscible solution. This patent does not suggest or disclose the use of diaryl methylene monomer containing at least 1 amine radical on each aryl nucleus as one reactant in the formation of the desired membrane. Likewise, U.S. Pat. No. 4,612,118 discloses a semipermeable membrane comprising a composite of polyamine and triazine. The polyamine monomers which are utilized as one component of the membrane are not employed as a free amine. Rather, this amine is allowed to react with acryl triazines to form Michael-type adducts which are then utilized as the monomers in the membrane forming process. The resulting membranes are of the polyurea type, the urea group of which being generated by the reaction of the triazine Michael adducts with polyisocyanates, the latter being the other component of the membrane system. The polyurea type membranes which are formed by the process disclosed in this patent comprise a different chemical class having different physical and chemical properties than the polyamide membranes of the present invention.
We have now discovered that saline water may be desalinated by passage through a semipermeable membrane which possesses the desirable characteristics of high salt rejection, good flux, and high resistance to chlorine, said membrane being prepared in an interfacially polymerized reaction utilizing a diaryl methane monomer containing at least 1 amine radical on each aryl nucleus and aromatic carboxylic acid chloride as the reactive components.